Conventional draglines include a dragline housing which incorporates a hoist winch, an outwardly extending boom carrying a hoist cable connected to the hoist winch which may extend downwardly from an outer end of the boom attached so an excavator bucket as hereinafter described and a drag cable attached to the excavator bucket which is connected to a drag winch located in the dragline hoising. Both the drag winch and hoist winch are suitably driven from electric motors and the dragline housing is suitably rotatably mounted to a support base by a central bearing. The dragline housing may be equipped with crawler tracks for locomotion or alternatively and more commonly is equipped with a walking mechanism having a pair of support feet which are reciprocatable relative to the dragline housing and thus may propel the dragline housing in a desired direction of travel.
The excavator bucket is usually arcuate in shape having a base and upper side wall provided with an open front or access mouth whereby soil or overburden may gain access to the interior of the bucket as the bucket is dragged or moved through loose overburden by the drag cable(s). A plurality of ripper teeth along a free edge of the base may be provided in the open front or access mouth.
The rear end of the excavator bucket is provided with a pair of opposed support chains each attached to a respective support lug on the external face of the upper side wall. The chains may be interconnected by a horizontal bar. The hoist cable(s) may be split into two half portions or hoist cable components wherein each component is attached to an associated end of the horizontal bar.
There also may be provided a support loop adjacent the open front of the bucket to which may be attached further cable(s) connected to the hoist cable(s) and also to the drag cable(s) by use of an appropriate sheave mechanism. There also may be provided a pair of drag chains attached to each front vertical edge of the upper wall which are each attached to the drag cable(s).
The abovementioned conventional dragline operates to remove overburden or to mine a mineral from a valuable mineral or coal deposit such as a coal seam. After initially drilling and blasting the overburden by a suitable explosive such as ammonium nitrate, the overburden is loosened and softened. A bulldozer was then used to level a portion of the overburden to provide a flattened hill for supporting the dragline housing. The excavator bucket was then moved to the desired location wherein the drag winch was free spinning so that the drag cable was slack. In this movement the hoist winch has enough tension to hold the excavator bucket as it swings freely when suspended from the boom. Then the excavator bucket is placed on the ground by actuation of the hoist cable and tension is subsequently applied to the drag cables. The bucket is subsequently dragged through the loose overburden until it is full. The tension is then maintained in the drag cables and tension is applied to the hoist cables to elevate the full bucket above the overburden. By balanced control of the hoist winch and drag winch the bucket may then be moved towards the top of the boom which has a sheave for supporting the hoist cable. The dragline housing is then rotated away from the excavation site towards a spoil pile or dump from the overburden in the bucket and is tipped out of the excavator bucket by slackening of the tension in the drag cables. By appropriate control of the hoist cable the empty excavator bucket then assumes its normal orientation and is then moved back to the excavation site by rotation of the dragline housing.
The abovedescribed conventional dragline apparatus had several disadvantages and one major disadvantage was that it was constrained by geometric or physical size parameters such as the length of the hoist cable and drag cable(s) and/or the length of the boom excavation site. Thus for example, if a coal seam was too deep for the cables then "rehandling operations" were necessary wherein material already removed or dug by the excavator bucket is dug again by auxiliary or companion excavation equipment such as a bucket wheel excavator or repositioning of the dragline. Thus in relation to a particular mine if the coal seam was too deep for the physical parameters of the machine it was necessary to carry out a prestripping operation by truck and shovel, bucket wheel excavator or scraper to remove the top level of overburden. Rehandling or prestripping operations were found to be very expensive in practice and insufficient in relation to the depth of the open cut excavation site. Thus in other words, as the depth increased, there was a tendency for the spoil pile and high wall excavation material separating the excavation site and the flattened hill supporting the dragline housing to subside and fall into the open cut excavation site.
It therefore is an object of the invention to provide a dragline apparatus and method of use which may alleviate the abovementioned disadvantages associated with the prior art.